Patterns of emigration, immigration, and colony composition during a population crash are described for the plains vizcacha (Lagostomus maximus) in La Pampa, Argentina. The sex ratio of young-of-the-year was slightly male-biased and significantly different from the female-biased sex ratio of adults. Each social group lived in a communal burrow system. Females were recruited into their natal burrow system. Males remained in their natal burrow system up to 27 months, but did not breed there. Tenure of adult females within a social group was longer than tenure of adult males. Females and young moved to other burrow systems as a group, and only when the number of animals became low in their own social group. In contrast, solitary adult males moved into and out of social groups, even when the number of animals in the group was high. Adult males never were present in a social group for more than one breeding season. Inter-group movements occurred in all seasons except winter in both sexes. Dispersal patterns during a population decline, particularly the delay in dispersal of young males and high turnover of individuals within social groups, may differ from movement patterns in habitats with more abundant and stable resources.

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